Monthly Notices of the Royal Astronomical Society, Jan 13, 2017
Dust formation and resulting mass loss around Asymptotic Giant Branch (AGB) stars with initial me... more Dust formation and resulting mass loss around Asymptotic Giant Branch (AGB) stars with initial metallicity in the range of 0 ≤ Z ini ≤ 10 -4 and initial mass 2 ≤ M ini /M ⊙ ≤ 5 are explored by the hydrodynamical calculations of dust-driven wind (DDW) along the AGB evolutionary tracks. We employ the MESA code to simulate the evolution of stars, assuming an empirical mass-loss rate in the post-main sequence phase, and considering the three types of low-temperature opacities (scaled-solar, CO-enhanced, and CNO-enhanced opacities) to elucidate the effect on the stellar evolution and the DDW. We find that the treatment of low-temperature opacity strongly affects the dust formation and resulting DDW; in the carbon-rich AGB phase, the maximum Ṁ of M ini ≥ 3 M ⊙ star with the CO-enhanced opacity is at least one order of magnitude smaller than that with the CNO-enhanced opacity. A wide range of stellar parameters being covered, a necessary condition for driving efficient DDW with Ṁ ≥ 10 -6 M ⊙ yr -1 is expressed as the effective temperature T eff < ∼ 3850 K and log(δ C L/κ R M) > ∼ 10.43 logT eff -32.33 with the carbon excess δ C defined as ε C -ε O and the Rosseland mean opacity κ R in units of cm 2 g -1 in the surface layer, and the stellar mass (lu- minosity) M (L) in solar units. The derived fitting formulae of gas and dust mass-loss rates in terms of input stellar parameters could be useful for investigating the dust yield from AGB stars in the early Universe being consistent with the stellar evolution calculations.
Optical properties of dust aggregates of astrophysical interest have been investigated by means o... more Optical properties of dust aggregates of astrophysical interest have been investigated by means of the discrete dipole approximation (DDA) to clarify how differences in structure, size and chemical composition of dust aggregates affect their optical properties. Two types of dust aggregates have been treated; one is the Ballistic Particle-Cluster Aggregates (BPCA) whose fractal dimension D '" 3, and the other is the Ballistic Cluster-Cluster Aggregates (BCCA) for which D '" 2. Two kinds of minerals have been considered for the constituent particles; silicate and magnetite. The results of the calculations show that the optical properties of dust aggregates are divided into two regimes according to their characteristic size ae; in the wavelength range A ~ Ae = 2nae, the optical properties are qualitatively reproduced by the Rayleigh approximation for a sphere with the same mass and do not depend on the structure of the dust aggregates. The scattering cross sections start to deviate from the Rayleigh regime at about the characteristic wavelength Ac = 2nae, which is a diagnostic for the sizes of dust aggregates, irrespective of the chemical composition of the constituent particles. The wavelength dependence of the absorption cross sections can be interpreted in terms of the optical thickness of the dust aggregates themselves and can be approximated with a simple formula. This formula is useful to determine the dependence of the temperature of dust aggregates in space on their sizes as well as structures, which plays a crucial role in considering the physical and chemical evolution of dust aggregates in space. The wavelength dependence of the asymmetry parameter reflects the size as well as the structure of dust aggregates, independent of the chemical composition of the constituent particles. BPCA are characterized by an enhanced forward scattering in the region of A <: Ae in comparison with BCCA with the same mass, in spite of the smaller size of BPCA. The applicability of the Maxwell-Garnett mixing rule to dust aggregates is investigated in comparison with the results of the calculations using the discrete dipole approximation.
Polarized infrared reflectance of large single crystals of forsterite was measured for each cryst... more Polarized infrared reflectance of large single crystals of forsterite was measured for each crystal axis at frequencies between 5000 cm -1 and 100 cm -1 with a resolution of 1 cm -1 . The reflectance spectra were analyzed based on classical dispersion theory. A set of oscillator parameters for crystalline axes of forsterite was obtained. Those parameters are used to derive optical constants of forsterite, which are important for analyses of observed spectra of astronomical objects and laboratory spectra of particle samples. Calculations of absorption for small particles using the bulk optical constants are compared with mid-infrared and far-infrared absorption spectra measured in the laboratory.
Based on Mie theory and Maxwell-Garnett effective medium theory (MG-Mie), we have examined a rati... more Based on Mie theory and Maxwell-Garnett effective medium theory (MG-Mie), we have examined a ratio P of the radiation pressure forces to the gravity on porous aggregates with fractal structures, which consist of absorbing (graphite or magnetite) or dielectric (silicate) materials. A comparison of P by MG-Mie theory with those computed by the discreate dipole approximation (DDA) has shown that when the dust aggregates have rather compact structures and/or the constituent particles are absorbing materials, the values of P by DDA theory are reproduced reasonably by MG-Mie theory. Furthermore, it is found that (i) the higher the porosity, the smaller the value of P becomes near the maximum of P at a characteristic radius of the aggregate of about 0.1Ilm. (ii) For very porous aggregates, P does not depend on size in contrast to a remarkable size dependence of pfor compact spheres. (iii) It would be expected from the MG-Mie theory that regardless of the chemical composition of constituents, P of porous grains approaches a constant value, which is close to that for the individual constituent particle, as the porosity increases. Consequently, when the radiation pressure forces are significant in comparison with the gravity, the dynamical behaviour oflarge fluffy particles with high porosity in the interplanetary space seems to be similar to that of the small compact particles which make up their internal structure.
Observations have confirmed the formation of dust grains in the metal-rich ejecta of SN 1987A. In... more Observations have confirmed the formation of dust grains in the metal-rich ejecta of SN 1987A. In this paper the grain formation in the ejecta is reinvestigated on the basis of the revised hydrodynamical model and elemental composition of the ejecta, and of the theory of homogeneous nucleation and grain growth. The adopted abundance distribution in the ejecta, inferred from the behavior of the bolo metric light curve around its maximum and the early emergence of X-rays and )I-rays, results in the sequential formation of Alz0 3 , MgSi03 and Fe30 4 grains respectively in the ejecta at 1.0 M 0 :S; Mr :s;4.4 M0 as the gas cools down. In the inner region at Mr:S; 1.0 M 0 , on the other hand, the latent heat deposited during the grain growth retards the formation of dust grains. The observed enhancement of 10 Ilm flux at -day 465 after the explosion being assigned to the thermal radiation from Al z 0 3 grains formed in the ejecta, MgSi0 3 grains start to form at -day 550, and Fe30 4 grains at -day 620, and the grains cease to form at -day 730 in the region at 1.0 M 0 :s; Mr:S; 4.4 M 0' The total mass of dust grains formed in this region is -0.23 M 0' The radii of grains newly formed in the ejecta are typically -10 A for Al z 0 3 and Fe 3 0 4 , and -70 A for MgSi0 3 grains. The calculated grain radius smaller than 0.01 Ilm satisfies the constraint on the grain size in the ejecta imposed by the analysis of the blue-shifted optical lines. Comparing the observed infrared light curve with the calculated one based on both the condensation calculation and the observed 10 Ilm flux, we suggest that: 1) The condensation efficiency of dust grains in the ejecta is greater than 0.3, or the clumps occupies the fraction of at least -20% of the ejecta on the average. 2) The grain number density in the clumps is less than 5 times that of the uniform ejecta. 3) The observed infrared luminosity is attributed to the thermal radiation from dust grains in the optically thick part of the ejecta.
We propose that the so-called 11.3p,m emission features observed toward carbon stars can be attri... more We propose that the so-called 11.3p,m emission features observed toward carbon stars can be attributed to small spherical core-mantle type grains consisting of a SiC core and a carbon mantle or small carbon grains containing SiC as an impurity. Based on the optical constants measured from bulk a-SiC, we show that as the volume fraction of the carbon mantle increases, the peak wavelength of the absorption shifts towards longer wavelengths, the strength decreases, and the band width becomes wider; the emission peak appears at 11.3p,m for a volume fraction of 0.4. This behaviour would explain a variety of SiC features observed toward infrared carbon stars. The possibility for the formation of the proposed coremantle dust grains in circumstellar envelopes of carbon stars has been investigated by applying a theory of homogeneous nucleation and grain growth. Using a simple model for circumstellar envelopes, we demonstrate that the nucleation of SiC grains always precedes that of carbon grains when the non-LTE effect, i.e. the difference between the temperatures of gas and small clusters, is taken into account. In the case of mass loss rates IMI ;S 1.5 x 10-5 Mdyr, the SiC grains are gradually coated by carbon and show a distinctive emission feature around 11.3p,m. Otherwise, the condensation of SiC and carbon simultaneously occurs after an avalanche of the formation of SiC nuclei and the emission feature is almost suppressed for IMI 2: 1.5 x 10-5 Mev/yr. The radiation pressure force acting on these dust grains is sufficient to drive the mass loss of carbon stars when the volume fraction of carbon is larger than 0.1. The profile of the 11.3p,m feature can be a diagnostic mean of the formation process of dust grains prevailing in circumstellar envelopes of carbon stars and well reflects the differences in the mass loss rate and the evolutionary stage.
We used the Spitzer Space Telescope's Infrared Spectrograph to create a high resolution spectral ... more We used the Spitzer Space Telescope's Infrared Spectrograph to create a high resolution spectral map of the central region of the Cassiopeia A supernova remnant, allowing us to make a Doppler reconstruction of its 3D structure. The ejecta responsible for this emission have not yet encountered the remnant's reverse shock or the circumstellar medium, making it an ideal laboratory for exploring the dynamics of the supernova explosion itself. We observe that the O, Si, and S ejecta can form both sheet-like structures as well as filaments. Si and O, which come from different nucleosynthetic layers of the star, are observed to be coincident in velocity space in some regions, and separated by 500 km s -1 or more in others. Ejecta traveling toward us are, on average, ∼900 km s -1 slower than the material traveling away from us. We compare our observations to recent supernova explosion models and find that no single model can simultaneously reproduce all the observed features. However, models of different supernova explosions can collectively produce the observed geometries and structures of the interior emission. We use the results from the models to address the conditions during the supernova explosion, concentrating on asymmetries in the shock structure. We also predict that the back surface of Cassiopeia A will begin brightening in ∼30 years, and the front surface in ∼100 years.
Optical properties of dust aggregates of astrophysical interest have been investigated by means o... more Optical properties of dust aggregates of astrophysical interest have been investigated by means of the discrete dipole approximation (DDA) to clarify how differences in structure, size and chemical composition of dust aggregates affect their optical properties. Two types of dust aggregates have been treated; one is the Ballistic Particle-Cluster Aggregates (BPCA) whose fractal dimension D∼3, and the other is the Ballistic Cluster-Cluster Aggregates (BCCA) for which D∼2. Two kinds of minerals have been considered for the constituent particles; silicate and magnetite
We present a theoretical model for Type Ib supernova (SN) 2006jc associated with a luminous blue ... more We present a theoretical model for Type Ib supernova (SN) 2006jc associated with a luminous blue variable (LBV)-like event. We calculate the presupernova evolution of the progenitor star, hydrodynamics and nucleosynthesis of the SN explosion, and the SN bolometric light curve (LC). The observed bolometric LC is constructed by integrating the UV, optical, near-infrared (NIR), and midinfrared (MIR) fluxes. The progenitor is assumed to be as massive as 40M⊙ on the zero-age. The star undergoes extensive mass loss to reduce its mass down to as small as 6.9M⊙, thus becoming a WC Wolf-Rayet star at the presupernova stage. The WC star model has a thick carbon-rich layer, in which amorphous carbon grains can be formed during the explosion. This could explain the brightening in the NIR flux and the observed dust feature in MIR. The typical main-sequence mass of a WC WolfRayet star and thus the progenitor of SN 2006jc is more massive than 40M⊙. We suggest that the explosions of stars more mass...
Monthly Notices of the Royal Astronomical Society, 2017
Dust formation and resulting mass loss around Asymptotic Giant Branch (AGB) stars with initial me... more Dust formation and resulting mass loss around Asymptotic Giant Branch (AGB) stars with initial metallicity in the range of 0 ≤ Z ini ≤ 10 -4 and initial mass 2 ≤ M ini /M ⊙ ≤ 5 are explored by the hydrodynamical calculations of dust-driven wind (DDW) along the AGB evolutionary tracks. We employ the MESA code to simulate the evolution of stars, assuming an empirical mass-loss rate in the post-main sequence phase, and considering the three types of low-temperature opacities (scaled-solar, CO-enhanced, and CNO-enhanced opacities) to elucidate the effect on the stellar evolution and the DDW. We find that the treatment of low-temperature opacity strongly affects the dust formation and resulting DDW; in the carbon-rich AGB phase, the maximum Ṁ of M ini ≥ 3 M ⊙ star with the CO-enhanced opacity is at least one order of magnitude smaller than that with the CNO-enhanced opacity. A wide range of stellar parameters being covered, a necessary condition for driving efficient DDW with Ṁ ≥ 10 -6 M ⊙ yr -1 is expressed as the effective temperature T eff < ∼ 3850 K and log(δ C L/κ R M) > ∼ 10.43 logT eff -32.33 with the carbon excess δ C defined as ε C -ε O and the Rosseland mean opacity κ R in units of cm 2 g -1 in the surface layer, and the stellar mass (lu- minosity) M (L) in solar units. The derived fitting formulae of gas and dust mass-loss rates in terms of input stellar parameters could be useful for investigating the dust yield from AGB stars in the early Universe being consistent with the stellar evolution calculations.
Symposium - International Astronomical Union, 1999
Crystallization of silicate has been investigated within the framework of dust formation in stead... more Crystallization of silicate has been investigated within the framework of dust formation in steady state gas outflows around oxygen–rich AGB stars, where silicates are locked not only into homogeneous silicate grains but also into the mantles of heterogeneous grains. Based on the thermal history of dust grains after their formation, the crystallization calculation results in no crystalline silicate for the mass loss rateṀ≤ 2 × 10−5M⊙yr−1. Only silicate in the mantles of heterogeneous grains can be crystallized forṀ≥ 3 × 10−5M⊙yr−1, while homogeneous silicate grains remain amorphous. The mass fraction of crystalline silicate increases with increasingṀ. The radiation transfer calculations confirm the appearance of an emission feature around 33.5 μm, taking olivine as a representative of crystalline silicates. On the other hand, the 10μm feature appears in absorption, being dominated by homogeneous silicate grains. These trends are consistent with the observations. Thus the crystalline...
We present infrared multi-epoch observations of the dust forming nova V1280 Sco over ∼2000 days f... more We present infrared multi-epoch observations of the dust forming nova V1280 Sco over ∼2000 days from the outburst. The temporal evolution of the infrared spectral energy distributions at 1272, 1616 and 1947 days can be explained by
Conditions for formation and preservation of amorphous ice formed through condensation of water v... more Conditions for formation and preservation of amorphous ice formed through condensation of water vapor on a substrate is investigated both theoretically and experimentally. The kinetic consideration of deposition of vapor leads to the condition for formation of amorphous ice on a cold substrate: the flux of water vapor onto the substrate should be larger than a critical flux Ds/a 4 == Fe, where Ds is the surface diffusion coefficient of water molecules on the substrate and a the lattice constant of the substrate. The validity of the derived condition has been confirmed by measuring the critical flux of water vapor onto a substrate of polycrystalline cubic ice as a function of temperature. From the measured Fe, a surface diffusion coefficient of H20 molecules on polycrystalline cubic ice has been determined. With regard to the preservation condition, we derive a theoretical expression of the time scale te for crystallization of amorphous ice, which has the same form at low temperatures as the empirical formula used so far. A self-diffusion coefficient of amorphous ice is estimated from a comparison between the theoretical and empirical formulas of te. The results are applied to condensation of ice in molecular clouds, circum stellar envelopes of late-type stars, and the primordial solar nebula. Discussion is given on the crystallinity of ices in these sites.
Mie theory and Maxwell-Garnett effective medium theory (MG-Mie) are used to examine a ratio, beta... more Mie theory and Maxwell-Garnett effective medium theory (MG-Mie) are used to examine a ratio, beta, of the radiation pressure forces to the gravity on porous aggregates with fractal structures, which consist of absorbing or dielectric materials. A comparison of beta by Mg-Mie theory with those computed by the discrete dipole approximation (DDA) shows that when the dust aggregates have rather
We used the Spitzer Space Telescope's Infrared Spectrograph to map nearly the entire extent of Ca... more We used the Spitzer Space Telescope's Infrared Spectrograph to map nearly the entire extent of Cassiopeia A between 5 and 40 μm. Using infrared and Chandra X-ray Doppler velocity measurements, along with the locations of optical ejecta beyond the forward shock, we constructed a three-dimensional model of the remnant. The structure of Cas A can be characterized into a spherical component, a tilted thick disk, and multiple ejecta jets/pistons and optical fast-moving knots all populating the thick disk plane. The Bright Ring in Cas A identifies the intersection between the thick plane/pistons and a roughly spherical reverse shock. The ejecta pistons indicate a radial velocity gradient in the explosion. Some ejecta pistons are bipolar with oppositely directed flows about the expansion center while some ejecta pistons show no such symmetry. Some ejecta pistons appear to maintain the integrity of the nuclear burning layers while others appear to have punched through the outer layers. The ejecta pistons indicate a radial velocity gradient in the explosion. In three dimensions, the Fe jet in the southeast occupies a "hole" in the Si-group emission and does not represent "overturning," as previously thought. Although interaction with the circumstellar medium affects the detailed appearance of the remnant and may affect the visibility of the southeast Fe jet, the bulk of the symmetries and asymmetries in Cas A are intrinsic to the explosion.
We investigate the formation of silicon carbide (SiC) grains in the framework of dustdriven wind ... more We investigate the formation of silicon carbide (SiC) grains in the framework of dustdriven wind around pulsating carbon-rich Asymptotic Giant Branch (C-rich AGB) stars in order to reveal not only the amount but also the size distribution. Two cases are considered for the nucleation process; one is the LTE case where the vibration temperature of SiC clusters T v is equal to the gas temperature as usual, and another is the non-LTE case in which T v is assumed to be the same as the temperature of small SiC grains. The results of hydrodynamical calculations for a model with stellar parameters of mass M * =1.0 M ⊙ , luminosity L * =10 4 L ⊙ , effective temperature T eff =2600 K, C/O ratio=1.4, and pulsation period P =650 days show the followings: In the LTE case, SiC grains condense in accelerated outflowing gas after the formation of carbon grains and the resulting averaged mass ratio of SiC to carbon grains of ∼ 10 -8 is too small to reproduce the value of 0.01-0.3 inferred from the radiative transfer models. On the other hand, in the non-LTE case, the formation region of SiC grains is inner than and/or almost identical to that of carbon grains due to the so-called inverse greenhouse effect. The mass ratio of SiC to carbon grains averaged at the outer boundary ranges from 0.098 to 0.23 for the sticking probability α s =0.1-1.0. The size distributions with the peak at ∼ 0.2-0.3 µm in radius cover the range of size derived from the analysis of presolar SiC grains. Thus the difference between temperatures of small cluster and gas plays a crucial role in the formation process of SiC grains around C-rich AGB stars, and this aspect should be explored for the formation process of dust grains in astrophysical environments.
We present a 3-dimensional analysis of the supernova remnant Cassiopeia A using high resolution s... more We present a 3-dimensional analysis of the supernova remnant Cassiopeia A using high resolution spectra from the Spitzer Space Telescope. We observe supernova ejecta both immediately before and during the shock-ejecta interaction. We determine that the reverse shock of the remnant is spherical to within 7%, although the center of this sphere is offset from the geometric center of the remnant by 810 km s -1 . We determine that the velocity width of the nucleosynthetic layers is ∼1000 km s -1 over 4000 square arcsecond regions, although the velocity width of a layer along any individual line of sight is <250 km s -1 . Si and O, which come from different nucleosynthetic layers in the progenitor star, are observed to be coincident in velocity space in some directions, but segregated by up to ∼500 km s -1 in other directions. We compare these observations of the nucleosynthetic layers to predictions from supernova explosion models in an attempt to constrain such models. Finally, we observe small-scale, corrugated velocity structures that are likely caused during the supernova explosion itself, rather than hundreds of years later by dynamical instabilities at the remnant's reverse shock.
Observations have confirmed the formation of dust grains in the metal-rich ejecta of SN 1987A. In... more Observations have confirmed the formation of dust grains in the metal-rich ejecta of SN 1987A. In this paper the grain formation in the ejecta is reinvestigated on the basis of the revised hydrodynamical model and elemental composition of the ejecta, and of the theory of homogeneous nucleation and grain growth. The adopted abundance distribution in the ejecta, inferred from the behavior of the bolo metric light curve around its maximum and the early emergence of X-rays and )I-rays, results in the sequential formation of Alz0 3 , MgSi03 and Fe30 4 grains respectively in the ejecta at 1.0 M 0 :S; Mr :s;4.4 M0 as the gas cools down. In the inner region at Mr:S; 1.0 M 0 , on the other hand, the latent heat deposited during the grain growth retards the formation of dust grains. The observed enhancement of 10 Ilm flux at -day 465 after the explosion being assigned to the thermal radiation from Al z 0 3 grains formed in the ejecta, MgSi0 3 grains start to form at -day 550, and Fe30 4 grains at -day 620, and the grains cease to form at -day 730 in the region at 1.0 M 0 :s; Mr:S; 4.4 M 0' The total mass of dust grains formed in this region is -0.23 M 0' The radii of grains newly formed in the ejecta are typically -10 A for Al z 0 3 and Fe 3 0 4 , and -70 A for MgSi0 3 grains. The calculated grain radius smaller than 0.01 Ilm satisfies the constraint on the grain size in the ejecta imposed by the analysis of the blue-shifted optical lines. Comparing the observed infrared light curve with the calculated one based on both the condensation calculation and the observed 10 Ilm flux, we suggest that: 1) The condensation efficiency of dust grains in the ejecta is greater than 0.3, or the clumps occupies the fraction of at least -20% of the ejecta on the average. 2) The grain number density in the clumps is less than 5 times that of the uniform ejecta. 3) The observed infrared luminosity is attributed to the thermal radiation from dust grains in the optically thick part of the ejecta.
We present the global distribution of fine structure infrared line emission in the Cassiopeia A s... more We present the global distribution of fine structure infrared line emission in the Cassiopeia A supernova remnant using data from the Spitzer Space Telescope's Infrared Spectrograph. We identify emission from ejecta materials in the interior, prior to their encounter with the reverse shock, as well as from the post-shock bright ring. The global electron density increases by 100 at the shock to ∼ 10 4 cm -3 , providing evidence for strong radiative cooling. There is also a dramatic change in ionization state at the shock, with the fading of emission from low ionization interior species like [Si ii] giving way to [S iv] and, at even further distances, high-energy X-rays from hydrogenic silicon. Two compact, crescent-shaped clumps with highly enhanced neon abundance are arranged symmetrically around the central neutron star. These neon crescents are very closely aligned with the "kick" direction of the compact object from the remnant's expansion center, tracing a new axis of explosion asymmetry. They indicate that much of the apparent macroscopic elemental mixing may arise from different compositional layers of ejecta now passing through the reverse shock along different directions.
Monthly Notices of the Royal Astronomical Society, Jan 13, 2017
Dust formation and resulting mass loss around Asymptotic Giant Branch (AGB) stars with initial me... more Dust formation and resulting mass loss around Asymptotic Giant Branch (AGB) stars with initial metallicity in the range of 0 ≤ Z ini ≤ 10 -4 and initial mass 2 ≤ M ini /M ⊙ ≤ 5 are explored by the hydrodynamical calculations of dust-driven wind (DDW) along the AGB evolutionary tracks. We employ the MESA code to simulate the evolution of stars, assuming an empirical mass-loss rate in the post-main sequence phase, and considering the three types of low-temperature opacities (scaled-solar, CO-enhanced, and CNO-enhanced opacities) to elucidate the effect on the stellar evolution and the DDW. We find that the treatment of low-temperature opacity strongly affects the dust formation and resulting DDW; in the carbon-rich AGB phase, the maximum Ṁ of M ini ≥ 3 M ⊙ star with the CO-enhanced opacity is at least one order of magnitude smaller than that with the CNO-enhanced opacity. A wide range of stellar parameters being covered, a necessary condition for driving efficient DDW with Ṁ ≥ 10 -6 M ⊙ yr -1 is expressed as the effective temperature T eff < ∼ 3850 K and log(δ C L/κ R M) > ∼ 10.43 logT eff -32.33 with the carbon excess δ C defined as ε C -ε O and the Rosseland mean opacity κ R in units of cm 2 g -1 in the surface layer, and the stellar mass (lu- minosity) M (L) in solar units. The derived fitting formulae of gas and dust mass-loss rates in terms of input stellar parameters could be useful for investigating the dust yield from AGB stars in the early Universe being consistent with the stellar evolution calculations.
Optical properties of dust aggregates of astrophysical interest have been investigated by means o... more Optical properties of dust aggregates of astrophysical interest have been investigated by means of the discrete dipole approximation (DDA) to clarify how differences in structure, size and chemical composition of dust aggregates affect their optical properties. Two types of dust aggregates have been treated; one is the Ballistic Particle-Cluster Aggregates (BPCA) whose fractal dimension D '" 3, and the other is the Ballistic Cluster-Cluster Aggregates (BCCA) for which D '" 2. Two kinds of minerals have been considered for the constituent particles; silicate and magnetite. The results of the calculations show that the optical properties of dust aggregates are divided into two regimes according to their characteristic size ae; in the wavelength range A ~ Ae = 2nae, the optical properties are qualitatively reproduced by the Rayleigh approximation for a sphere with the same mass and do not depend on the structure of the dust aggregates. The scattering cross sections start to deviate from the Rayleigh regime at about the characteristic wavelength Ac = 2nae, which is a diagnostic for the sizes of dust aggregates, irrespective of the chemical composition of the constituent particles. The wavelength dependence of the absorption cross sections can be interpreted in terms of the optical thickness of the dust aggregates themselves and can be approximated with a simple formula. This formula is useful to determine the dependence of the temperature of dust aggregates in space on their sizes as well as structures, which plays a crucial role in considering the physical and chemical evolution of dust aggregates in space. The wavelength dependence of the asymmetry parameter reflects the size as well as the structure of dust aggregates, independent of the chemical composition of the constituent particles. BPCA are characterized by an enhanced forward scattering in the region of A <: Ae in comparison with BCCA with the same mass, in spite of the smaller size of BPCA. The applicability of the Maxwell-Garnett mixing rule to dust aggregates is investigated in comparison with the results of the calculations using the discrete dipole approximation.
Polarized infrared reflectance of large single crystals of forsterite was measured for each cryst... more Polarized infrared reflectance of large single crystals of forsterite was measured for each crystal axis at frequencies between 5000 cm -1 and 100 cm -1 with a resolution of 1 cm -1 . The reflectance spectra were analyzed based on classical dispersion theory. A set of oscillator parameters for crystalline axes of forsterite was obtained. Those parameters are used to derive optical constants of forsterite, which are important for analyses of observed spectra of astronomical objects and laboratory spectra of particle samples. Calculations of absorption for small particles using the bulk optical constants are compared with mid-infrared and far-infrared absorption spectra measured in the laboratory.
Based on Mie theory and Maxwell-Garnett effective medium theory (MG-Mie), we have examined a rati... more Based on Mie theory and Maxwell-Garnett effective medium theory (MG-Mie), we have examined a ratio P of the radiation pressure forces to the gravity on porous aggregates with fractal structures, which consist of absorbing (graphite or magnetite) or dielectric (silicate) materials. A comparison of P by MG-Mie theory with those computed by the discreate dipole approximation (DDA) has shown that when the dust aggregates have rather compact structures and/or the constituent particles are absorbing materials, the values of P by DDA theory are reproduced reasonably by MG-Mie theory. Furthermore, it is found that (i) the higher the porosity, the smaller the value of P becomes near the maximum of P at a characteristic radius of the aggregate of about 0.1Ilm. (ii) For very porous aggregates, P does not depend on size in contrast to a remarkable size dependence of pfor compact spheres. (iii) It would be expected from the MG-Mie theory that regardless of the chemical composition of constituents, P of porous grains approaches a constant value, which is close to that for the individual constituent particle, as the porosity increases. Consequently, when the radiation pressure forces are significant in comparison with the gravity, the dynamical behaviour oflarge fluffy particles with high porosity in the interplanetary space seems to be similar to that of the small compact particles which make up their internal structure.
Observations have confirmed the formation of dust grains in the metal-rich ejecta of SN 1987A. In... more Observations have confirmed the formation of dust grains in the metal-rich ejecta of SN 1987A. In this paper the grain formation in the ejecta is reinvestigated on the basis of the revised hydrodynamical model and elemental composition of the ejecta, and of the theory of homogeneous nucleation and grain growth. The adopted abundance distribution in the ejecta, inferred from the behavior of the bolo metric light curve around its maximum and the early emergence of X-rays and )I-rays, results in the sequential formation of Alz0 3 , MgSi03 and Fe30 4 grains respectively in the ejecta at 1.0 M 0 :S; Mr :s;4.4 M0 as the gas cools down. In the inner region at Mr:S; 1.0 M 0 , on the other hand, the latent heat deposited during the grain growth retards the formation of dust grains. The observed enhancement of 10 Ilm flux at -day 465 after the explosion being assigned to the thermal radiation from Al z 0 3 grains formed in the ejecta, MgSi0 3 grains start to form at -day 550, and Fe30 4 grains at -day 620, and the grains cease to form at -day 730 in the region at 1.0 M 0 :s; Mr:S; 4.4 M 0' The total mass of dust grains formed in this region is -0.23 M 0' The radii of grains newly formed in the ejecta are typically -10 A for Al z 0 3 and Fe 3 0 4 , and -70 A for MgSi0 3 grains. The calculated grain radius smaller than 0.01 Ilm satisfies the constraint on the grain size in the ejecta imposed by the analysis of the blue-shifted optical lines. Comparing the observed infrared light curve with the calculated one based on both the condensation calculation and the observed 10 Ilm flux, we suggest that: 1) The condensation efficiency of dust grains in the ejecta is greater than 0.3, or the clumps occupies the fraction of at least -20% of the ejecta on the average. 2) The grain number density in the clumps is less than 5 times that of the uniform ejecta. 3) The observed infrared luminosity is attributed to the thermal radiation from dust grains in the optically thick part of the ejecta.
We propose that the so-called 11.3p,m emission features observed toward carbon stars can be attri... more We propose that the so-called 11.3p,m emission features observed toward carbon stars can be attributed to small spherical core-mantle type grains consisting of a SiC core and a carbon mantle or small carbon grains containing SiC as an impurity. Based on the optical constants measured from bulk a-SiC, we show that as the volume fraction of the carbon mantle increases, the peak wavelength of the absorption shifts towards longer wavelengths, the strength decreases, and the band width becomes wider; the emission peak appears at 11.3p,m for a volume fraction of 0.4. This behaviour would explain a variety of SiC features observed toward infrared carbon stars. The possibility for the formation of the proposed coremantle dust grains in circumstellar envelopes of carbon stars has been investigated by applying a theory of homogeneous nucleation and grain growth. Using a simple model for circumstellar envelopes, we demonstrate that the nucleation of SiC grains always precedes that of carbon grains when the non-LTE effect, i.e. the difference between the temperatures of gas and small clusters, is taken into account. In the case of mass loss rates IMI ;S 1.5 x 10-5 Mdyr, the SiC grains are gradually coated by carbon and show a distinctive emission feature around 11.3p,m. Otherwise, the condensation of SiC and carbon simultaneously occurs after an avalanche of the formation of SiC nuclei and the emission feature is almost suppressed for IMI 2: 1.5 x 10-5 Mev/yr. The radiation pressure force acting on these dust grains is sufficient to drive the mass loss of carbon stars when the volume fraction of carbon is larger than 0.1. The profile of the 11.3p,m feature can be a diagnostic mean of the formation process of dust grains prevailing in circumstellar envelopes of carbon stars and well reflects the differences in the mass loss rate and the evolutionary stage.
We used the Spitzer Space Telescope's Infrared Spectrograph to create a high resolution spectral ... more We used the Spitzer Space Telescope's Infrared Spectrograph to create a high resolution spectral map of the central region of the Cassiopeia A supernova remnant, allowing us to make a Doppler reconstruction of its 3D structure. The ejecta responsible for this emission have not yet encountered the remnant's reverse shock or the circumstellar medium, making it an ideal laboratory for exploring the dynamics of the supernova explosion itself. We observe that the O, Si, and S ejecta can form both sheet-like structures as well as filaments. Si and O, which come from different nucleosynthetic layers of the star, are observed to be coincident in velocity space in some regions, and separated by 500 km s -1 or more in others. Ejecta traveling toward us are, on average, ∼900 km s -1 slower than the material traveling away from us. We compare our observations to recent supernova explosion models and find that no single model can simultaneously reproduce all the observed features. However, models of different supernova explosions can collectively produce the observed geometries and structures of the interior emission. We use the results from the models to address the conditions during the supernova explosion, concentrating on asymmetries in the shock structure. We also predict that the back surface of Cassiopeia A will begin brightening in ∼30 years, and the front surface in ∼100 years.
Optical properties of dust aggregates of astrophysical interest have been investigated by means o... more Optical properties of dust aggregates of astrophysical interest have been investigated by means of the discrete dipole approximation (DDA) to clarify how differences in structure, size and chemical composition of dust aggregates affect their optical properties. Two types of dust aggregates have been treated; one is the Ballistic Particle-Cluster Aggregates (BPCA) whose fractal dimension D∼3, and the other is the Ballistic Cluster-Cluster Aggregates (BCCA) for which D∼2. Two kinds of minerals have been considered for the constituent particles; silicate and magnetite
We present a theoretical model for Type Ib supernova (SN) 2006jc associated with a luminous blue ... more We present a theoretical model for Type Ib supernova (SN) 2006jc associated with a luminous blue variable (LBV)-like event. We calculate the presupernova evolution of the progenitor star, hydrodynamics and nucleosynthesis of the SN explosion, and the SN bolometric light curve (LC). The observed bolometric LC is constructed by integrating the UV, optical, near-infrared (NIR), and midinfrared (MIR) fluxes. The progenitor is assumed to be as massive as 40M⊙ on the zero-age. The star undergoes extensive mass loss to reduce its mass down to as small as 6.9M⊙, thus becoming a WC Wolf-Rayet star at the presupernova stage. The WC star model has a thick carbon-rich layer, in which amorphous carbon grains can be formed during the explosion. This could explain the brightening in the NIR flux and the observed dust feature in MIR. The typical main-sequence mass of a WC WolfRayet star and thus the progenitor of SN 2006jc is more massive than 40M⊙. We suggest that the explosions of stars more mass...
Monthly Notices of the Royal Astronomical Society, 2017
Dust formation and resulting mass loss around Asymptotic Giant Branch (AGB) stars with initial me... more Dust formation and resulting mass loss around Asymptotic Giant Branch (AGB) stars with initial metallicity in the range of 0 ≤ Z ini ≤ 10 -4 and initial mass 2 ≤ M ini /M ⊙ ≤ 5 are explored by the hydrodynamical calculations of dust-driven wind (DDW) along the AGB evolutionary tracks. We employ the MESA code to simulate the evolution of stars, assuming an empirical mass-loss rate in the post-main sequence phase, and considering the three types of low-temperature opacities (scaled-solar, CO-enhanced, and CNO-enhanced opacities) to elucidate the effect on the stellar evolution and the DDW. We find that the treatment of low-temperature opacity strongly affects the dust formation and resulting DDW; in the carbon-rich AGB phase, the maximum Ṁ of M ini ≥ 3 M ⊙ star with the CO-enhanced opacity is at least one order of magnitude smaller than that with the CNO-enhanced opacity. A wide range of stellar parameters being covered, a necessary condition for driving efficient DDW with Ṁ ≥ 10 -6 M ⊙ yr -1 is expressed as the effective temperature T eff < ∼ 3850 K and log(δ C L/κ R M) > ∼ 10.43 logT eff -32.33 with the carbon excess δ C defined as ε C -ε O and the Rosseland mean opacity κ R in units of cm 2 g -1 in the surface layer, and the stellar mass (lu- minosity) M (L) in solar units. The derived fitting formulae of gas and dust mass-loss rates in terms of input stellar parameters could be useful for investigating the dust yield from AGB stars in the early Universe being consistent with the stellar evolution calculations.
Symposium - International Astronomical Union, 1999
Crystallization of silicate has been investigated within the framework of dust formation in stead... more Crystallization of silicate has been investigated within the framework of dust formation in steady state gas outflows around oxygen–rich AGB stars, where silicates are locked not only into homogeneous silicate grains but also into the mantles of heterogeneous grains. Based on the thermal history of dust grains after their formation, the crystallization calculation results in no crystalline silicate for the mass loss rateṀ≤ 2 × 10−5M⊙yr−1. Only silicate in the mantles of heterogeneous grains can be crystallized forṀ≥ 3 × 10−5M⊙yr−1, while homogeneous silicate grains remain amorphous. The mass fraction of crystalline silicate increases with increasingṀ. The radiation transfer calculations confirm the appearance of an emission feature around 33.5 μm, taking olivine as a representative of crystalline silicates. On the other hand, the 10μm feature appears in absorption, being dominated by homogeneous silicate grains. These trends are consistent with the observations. Thus the crystalline...
We present infrared multi-epoch observations of the dust forming nova V1280 Sco over ∼2000 days f... more We present infrared multi-epoch observations of the dust forming nova V1280 Sco over ∼2000 days from the outburst. The temporal evolution of the infrared spectral energy distributions at 1272, 1616 and 1947 days can be explained by
Conditions for formation and preservation of amorphous ice formed through condensation of water v... more Conditions for formation and preservation of amorphous ice formed through condensation of water vapor on a substrate is investigated both theoretically and experimentally. The kinetic consideration of deposition of vapor leads to the condition for formation of amorphous ice on a cold substrate: the flux of water vapor onto the substrate should be larger than a critical flux Ds/a 4 == Fe, where Ds is the surface diffusion coefficient of water molecules on the substrate and a the lattice constant of the substrate. The validity of the derived condition has been confirmed by measuring the critical flux of water vapor onto a substrate of polycrystalline cubic ice as a function of temperature. From the measured Fe, a surface diffusion coefficient of H20 molecules on polycrystalline cubic ice has been determined. With regard to the preservation condition, we derive a theoretical expression of the time scale te for crystallization of amorphous ice, which has the same form at low temperatures as the empirical formula used so far. A self-diffusion coefficient of amorphous ice is estimated from a comparison between the theoretical and empirical formulas of te. The results are applied to condensation of ice in molecular clouds, circum stellar envelopes of late-type stars, and the primordial solar nebula. Discussion is given on the crystallinity of ices in these sites.
Mie theory and Maxwell-Garnett effective medium theory (MG-Mie) are used to examine a ratio, beta... more Mie theory and Maxwell-Garnett effective medium theory (MG-Mie) are used to examine a ratio, beta, of the radiation pressure forces to the gravity on porous aggregates with fractal structures, which consist of absorbing or dielectric materials. A comparison of beta by Mg-Mie theory with those computed by the discrete dipole approximation (DDA) shows that when the dust aggregates have rather
We used the Spitzer Space Telescope's Infrared Spectrograph to map nearly the entire extent of Ca... more We used the Spitzer Space Telescope's Infrared Spectrograph to map nearly the entire extent of Cassiopeia A between 5 and 40 μm. Using infrared and Chandra X-ray Doppler velocity measurements, along with the locations of optical ejecta beyond the forward shock, we constructed a three-dimensional model of the remnant. The structure of Cas A can be characterized into a spherical component, a tilted thick disk, and multiple ejecta jets/pistons and optical fast-moving knots all populating the thick disk plane. The Bright Ring in Cas A identifies the intersection between the thick plane/pistons and a roughly spherical reverse shock. The ejecta pistons indicate a radial velocity gradient in the explosion. Some ejecta pistons are bipolar with oppositely directed flows about the expansion center while some ejecta pistons show no such symmetry. Some ejecta pistons appear to maintain the integrity of the nuclear burning layers while others appear to have punched through the outer layers. The ejecta pistons indicate a radial velocity gradient in the explosion. In three dimensions, the Fe jet in the southeast occupies a "hole" in the Si-group emission and does not represent "overturning," as previously thought. Although interaction with the circumstellar medium affects the detailed appearance of the remnant and may affect the visibility of the southeast Fe jet, the bulk of the symmetries and asymmetries in Cas A are intrinsic to the explosion.
We investigate the formation of silicon carbide (SiC) grains in the framework of dustdriven wind ... more We investigate the formation of silicon carbide (SiC) grains in the framework of dustdriven wind around pulsating carbon-rich Asymptotic Giant Branch (C-rich AGB) stars in order to reveal not only the amount but also the size distribution. Two cases are considered for the nucleation process; one is the LTE case where the vibration temperature of SiC clusters T v is equal to the gas temperature as usual, and another is the non-LTE case in which T v is assumed to be the same as the temperature of small SiC grains. The results of hydrodynamical calculations for a model with stellar parameters of mass M * =1.0 M ⊙ , luminosity L * =10 4 L ⊙ , effective temperature T eff =2600 K, C/O ratio=1.4, and pulsation period P =650 days show the followings: In the LTE case, SiC grains condense in accelerated outflowing gas after the formation of carbon grains and the resulting averaged mass ratio of SiC to carbon grains of ∼ 10 -8 is too small to reproduce the value of 0.01-0.3 inferred from the radiative transfer models. On the other hand, in the non-LTE case, the formation region of SiC grains is inner than and/or almost identical to that of carbon grains due to the so-called inverse greenhouse effect. The mass ratio of SiC to carbon grains averaged at the outer boundary ranges from 0.098 to 0.23 for the sticking probability α s =0.1-1.0. The size distributions with the peak at ∼ 0.2-0.3 µm in radius cover the range of size derived from the analysis of presolar SiC grains. Thus the difference between temperatures of small cluster and gas plays a crucial role in the formation process of SiC grains around C-rich AGB stars, and this aspect should be explored for the formation process of dust grains in astrophysical environments.
We present a 3-dimensional analysis of the supernova remnant Cassiopeia A using high resolution s... more We present a 3-dimensional analysis of the supernova remnant Cassiopeia A using high resolution spectra from the Spitzer Space Telescope. We observe supernova ejecta both immediately before and during the shock-ejecta interaction. We determine that the reverse shock of the remnant is spherical to within 7%, although the center of this sphere is offset from the geometric center of the remnant by 810 km s -1 . We determine that the velocity width of the nucleosynthetic layers is ∼1000 km s -1 over 4000 square arcsecond regions, although the velocity width of a layer along any individual line of sight is <250 km s -1 . Si and O, which come from different nucleosynthetic layers in the progenitor star, are observed to be coincident in velocity space in some directions, but segregated by up to ∼500 km s -1 in other directions. We compare these observations of the nucleosynthetic layers to predictions from supernova explosion models in an attempt to constrain such models. Finally, we observe small-scale, corrugated velocity structures that are likely caused during the supernova explosion itself, rather than hundreds of years later by dynamical instabilities at the remnant's reverse shock.
Observations have confirmed the formation of dust grains in the metal-rich ejecta of SN 1987A. In... more Observations have confirmed the formation of dust grains in the metal-rich ejecta of SN 1987A. In this paper the grain formation in the ejecta is reinvestigated on the basis of the revised hydrodynamical model and elemental composition of the ejecta, and of the theory of homogeneous nucleation and grain growth. The adopted abundance distribution in the ejecta, inferred from the behavior of the bolo metric light curve around its maximum and the early emergence of X-rays and )I-rays, results in the sequential formation of Alz0 3 , MgSi03 and Fe30 4 grains respectively in the ejecta at 1.0 M 0 :S; Mr :s;4.4 M0 as the gas cools down. In the inner region at Mr:S; 1.0 M 0 , on the other hand, the latent heat deposited during the grain growth retards the formation of dust grains. The observed enhancement of 10 Ilm flux at -day 465 after the explosion being assigned to the thermal radiation from Al z 0 3 grains formed in the ejecta, MgSi0 3 grains start to form at -day 550, and Fe30 4 grains at -day 620, and the grains cease to form at -day 730 in the region at 1.0 M 0 :s; Mr:S; 4.4 M 0' The total mass of dust grains formed in this region is -0.23 M 0' The radii of grains newly formed in the ejecta are typically -10 A for Al z 0 3 and Fe 3 0 4 , and -70 A for MgSi0 3 grains. The calculated grain radius smaller than 0.01 Ilm satisfies the constraint on the grain size in the ejecta imposed by the analysis of the blue-shifted optical lines. Comparing the observed infrared light curve with the calculated one based on both the condensation calculation and the observed 10 Ilm flux, we suggest that: 1) The condensation efficiency of dust grains in the ejecta is greater than 0.3, or the clumps occupies the fraction of at least -20% of the ejecta on the average. 2) The grain number density in the clumps is less than 5 times that of the uniform ejecta. 3) The observed infrared luminosity is attributed to the thermal radiation from dust grains in the optically thick part of the ejecta.
We present the global distribution of fine structure infrared line emission in the Cassiopeia A s... more We present the global distribution of fine structure infrared line emission in the Cassiopeia A supernova remnant using data from the Spitzer Space Telescope's Infrared Spectrograph. We identify emission from ejecta materials in the interior, prior to their encounter with the reverse shock, as well as from the post-shock bright ring. The global electron density increases by 100 at the shock to ∼ 10 4 cm -3 , providing evidence for strong radiative cooling. There is also a dramatic change in ionization state at the shock, with the fading of emission from low ionization interior species like [Si ii] giving way to [S iv] and, at even further distances, high-energy X-rays from hydrogenic silicon. Two compact, crescent-shaped clumps with highly enhanced neon abundance are arranged symmetrically around the central neutron star. These neon crescents are very closely aligned with the "kick" direction of the compact object from the remnant's expansion center, tracing a new axis of explosion asymmetry. They indicate that much of the apparent macroscopic elemental mixing may arise from different compositional layers of ejecta now passing through the reverse shock along different directions.
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Papers by takashi kozasa